Various automatic landing systems have become available during the past several years, often being incorporated in the aircraft's autopilot or flight control system. Basically, these automatic landing systems are digital signal processors that execute control laws that cause the aircraft to follow glideslope and localizer signals so that the aircraft approaches the destination runway without manual control by the pilot. When the aircraft reaches a suitable point in its descent, the automatic landing system initiates a flare maneuver to arrest aircraft rate of descent (sink rate) so that the aircraft touches down on the runway at a proper position and at a suitable sink rate. In the past, the point at which the automatic landing system initiates the flare maneuver has been based on the aircraft attaining a predetermined altitude. The flare initiation altitude used in these systems depends on the flight characteristics of the aircraft employing the system and is established to allow the automatic landing system to bring the aircraft to a safe landing in all situations in which the automatic landing system is used (i.e., for all ground speeds, aircraft landing weights, sink rates, etc., that are within the aircraft's automatic landing profile). For example, it has been typical practice to initiate an automatic landing flare maneuver when the aircraft landing gear is 50 or 45 feet above the ground, depending upon the particular aircraft of interest.
Initiating automatic landing flare maneuver at a predetermined altitude has certain disadvantages and drawbacks. For example, when an automatic landing is executed in a tail wind, with a heavily loaded aircraft, and/or is executed at a high-density altitude destination (which refers to the altitude of the destination airport adjusted to reflect ambient temperatures greater than standard atmospheric temperature), the aircraft groundspeed is higher than the nominal ground speed for the automatic landing procedure. This means that the aircraft will have a higher than nominal approach sink rate (i.e., will descend at a rate higher than it would during an approach at a nominal ground speed). To arrest the relatively high sink rate, it may be necessary for the automatic landing system to exert fairly aggressive control of the aircraft during the flare maneuver. Moreover, in situations in which the aircraft sink rate cannot be decreased to a low level, the aircraft may make a relatively hard landing. Although automatic landing systems in current use safely land the aircraft under relatively high ground speed approach conditions, the required flare maneuver and/or resulting hard landing may cause an undue degree of passenger concern, apprehension and discomfort.
Initiating an automatic landing flare maneuver at a predetermined altitude also is somewhat disadvantageous when the aircraft executes the automatic landing procedure at a relatively low ground speed because of head winds or a lightly loaded aircraft. When the aircraft ground speed is relatively low, the aircraft descends along the approach path with a lower than nominal approach sink rate. When this occurs, the automatic landing system may control the aircraft in a manner that results in relatively long flare time causing the aircraft crew and passengers to sometimes experience a "floating" sensation. This may cause the pilot to mistakenly believe that the landing will require more than normal runway distance (i.e., the aircraft will land "long"). Such a perception on the part of the pilot can cause unnecessary pilot concern and increases the possibility that the pilot will unnecessarily assume manual control of the aircraft, even when the landing is being executed under low visibility conditions.
For the above reasons, a need exists for methods and apparatus that initiate automatic landing flare maneuver in a manner that better accommodates higher than nominal and lower than nominal approach speeds (i.e., ground speeds that are within system limits, but higher or lower than optimal).